Field of the Invention
[0001] The present invention relates to method and apparatus for transmitting optical signals
in high-capacity, long distance, lightwave communication systems which use Return-To-Zero
(RZ) modulation format and dispersion management.
Background of the Invention
[0002] Optical nonlinearities of optical transmission fibers have become limiting factors
for long distance, high capacity lightwave transmission systems. In an optically amplified
transmission system transmission system, the amplified spontaneous emission (ASE)
noise degrades the signal-to-noise ratio (SNR), and a higher signal power is then
required to maintain a minimum SNR. However, optical nonlinearities distort the transmission
signal and thus limits the maximum'optical power that can be launched over the optical
transmission line.
[0003] It is possible to balance the self-phase-modulation (SPM) with chromatic dispersion
by a proper design of the transmission pulse waveform as well as the pulse energy.
Such nonlinear pulses are known as "optical solitons". Since the chromatic dispersion
is compensated for by optical nonlinearity, there is no need to perform dispersion
compensation in soliton systems. In this regard see, for example, U.S. Patent No.
4,558,921 (A. Hasegawa), issued on December 17, 1985, U.S. Patent No. 4,700,339 (J.P.
Gordon), issued on October 13, 1987, and U.S. Patent No. 5,035,481 (L.F. Mollenauer)
issued on July 30, 1991.
[0004] Although transoceanic soliton transmission is known, conventional soliton transmission
technology has not been commercialized. One of the major problems with conventional
soliton transmission is timing jitter. A soliton pulse width is typically approximately
10% of a bit period and there is no fundamental mechanism that can fix such short
pulses in time. Perturbations such as soliton-soliton interaction, frequency shift
due to the ASE noise, and an acoustic wave generated by a previous pulse tend to move
the pulses out of their original position as is indicated in U.S. Patent No. 5,710,649
(L.F. Mollenauer), issued on January 20, 1998. Still further, many techniques have
been used to reduce or eliminate timing jitter as is described in the book entitled
"Optical Fiber Telecommunications IIIA" by I. P. Kaminow et al., at pages 373-461
(Academic Press, 1997). All the above techniques, also known as "soliton control technologies"
are typically not cost effective in many practical applications, and also complicate
system designs.
[0005] Recently, a new class of solitons (dispersion managed solitons) were described in
the article entitled "Reduction of Gordon-Haus timing jitter by periodic dispersion
compensation in Soliton transmission" by M. Suzuki et al., Electronic Letters, Vol.
31, No. 23, pages 2027-2029, 1995. An article entitled "Soliton Transmission Using
Periodic Dispersion Compensation", by N. J. Smith et al. in Journal of Lightwave Technology,
Vol. 15, No. 10, pages 1808-1822, 1997, discusses a dispersion managed soliton (DMS)
which has been shown to have a much better performance than a conventional soliton,
while at the same time having inherent desired properties of conventional solitons
in dealing with optical nonlinearities. There are five major improvements in DMS transmission
systems compared to conventional soliton systems. These five improvements are (a)
energy enhancement wherein it is possible to launch much higher signal power for a
DMS signal than conventional soliton signals, which improves the system SNR; (b) reduced
timing jitter; (c) no additional soliton control technologies are required; (d) it
is compatible to existing non-return-to-zero (NRZ); and (e) it has high power and
dispersion tolerance.
[0006] In an article entitled "1 Tbit/s (100 x 10.7Gbits) Transoceanic Transmission Using
30nm-Wide Broadband Optical Repeaters with Aeff-Enlarged Positive Dispersion Slope
Fiber and Slope-Compensating DCF" by T. Tsuritani et al., at pages 38-39 of Post-Deadline
Papers,
25th European Conference on Optical Communications, 1999, discloses that significant system performance has been achieved using DMS technology
in many laboratory experiments in terms of both distance and total capacity. Similar
performance has also been demonstrated by many other such as, for example, the articles
entitled "1 Terabit/s WDM Transmission over 10,000km" by T. Naito et al., PD2-1, pages
24-25,
25th European Conference on Optical Communication, 1999, and "1.1-Tb/S (55 x 20-GB/s) DENSE WDM SOLITON TRANSMISSION OVER 3,020-km WIDELY-DISPERSON-MANAGED
TRANSMISSION LINE EMPLOYING 1.55/1.58 um HYBRID REPEATERS", by K. Fukuchi et al.,
PD2-10, pages 42-43,
25th European Conference on Optical Communication, 1999.
[0007] Although previous laboratory experiments have proven the feasibility of DMS systems
for long distance and high capacity applications, there are many challenges to achieve
both reliability and flexibility such that such DMS system can be used in a realistic
environment. For example, in terrestrial optical fiber networks, the distance between
repeaters or optical amplifiers can be as long as 130km. Such distances are significantly
longer than those of the previous experiments. The impacts of the longer repeater
spacing are two-fold. First, the required signal power is typically much higher to
overcome the SNR degradation caused by a required larger amplifier gain. Second, as
a result of the required higher signal power, optical nonlinearities become more important
for long distance transmission. Another challenge is that there are a variety of different
optical fiber types in existing optical fiber networks. Optical nonlinearities become
even more detrimental for a transmission line comprising different types of optical
fibers. As for DMS systems, the total capacity is limited not only by the optical
amplifier bandwidth, but also the higher order chromatic dispersion of the transmission
fiber. The situation is even more difficult when there are several different types
of optical fibers involved. A reliable and cost-effective solution to higher order
chromatic dispersion is one of the major challenges for high capacity long haul DMS
systems. Finally, distance and capacity are not the only requirements for next generation
optical networks. For example, it is highly desirable to have the flexibility to place
optical add/drop nodes anywhere along an optical transmission line.
[0008] In a dispersion managed soliton (DMS) system, DMS predicts significant power enhancement
which is valid for single channel propagation. The power enhancement cannot be fully
utilized for multi-channel system since the cross-phase modulation (XPM) dominates
the overall system performance. DMS systems further require an accurate balance between
the self-phase modulation (SPM) in the transmission fiber and the SPM a dispersion-compensating
fiber which often results in a much smaller system margin.
[0009] It is desirable to provide a high capacity ultra-long haul dispersion and nonlinearity
managed lightwave communication system which overcomes the problems described hereinabove.
Summary of the Invention
[0010] The present invention is directed to method and apparatus for transmitting optical
signals in high-capacity, long distance, lightwave communication systems which use
Return-To-Zero (RZ) modulation format and dispersion management.
[0011] Viewed from an apparatus aspect, the present invention is directed to an optical
transmission system. The optical transmission system comprises a transmitter terminal
and an optical transmission line. The transmitter terminal comprises a plurality of
return-to-zero (RZ) transmitters and a multiplexing arrangement. Each of the plurality
of return-to-zero (RZ) transmitters is adapted to receive a separate channel input
signal and to generate therefrom a corresponding forward error correction (FEC) modulated
output signal in a separate predetermined channel frequency sub-band of an overall
frequency band which includes a predetermined channel separation from an adjacent
channel frequency band generated by another RZ transmitter. The multiplexing arrangement
multiplexes the plurality of the predetermined channel frequency sub-bands from the
plurality of RZ transmitters into separate groups of frequency bands where the groups
of frequency bands have a predetermined band-gap separation therebetween wherein each
group of frequency bands has a predetermined separate pre-chirp introduced before
being multiplexed with all other groups of frequency bands into a single multiplexed
output signal. The optical transmission line has an input coupled to an output of
the multiplexing arrangement and is subdivided into a plurality of optical transmission
line sections. The optical transmission line comprises a plurality of Raman amplifiers
and at least one dispersion compensating line amplifier (DCLA). One of the plurality
of Raman amplifiers is located at the end of each optical transmission line section
and is adapted to receive at an input thereof the single multiplexed output signal
propagating in an associated transmission line section and to combine a predetermined
Raman pump power signal into the optical transmission line section in an opposite
direction to the received single multiplexed output signal to generate at an output
thereof an output signal which is Raman amplified for increasing a path averaged optical
power without increasing nonlinear degradation. The at least one DCLA is coupled to
an output of a Raman amplifier at the end of a predetermined group of optical transmission
line sections. The DCLA is adapted to introduce dispersion compensation for the single
multiplexed output signal, and to introduce separate high-order dispersion compensation
for each of the groups of frequency bands therein.
[0012] Viewed from a method aspect, the present invention is directed to a method of transmitting
signals in an optical transmission system. The method comprises the steps of:
(a) receiving each channel input signal of a plurality of channel input signals in
a separate one of a plurality of return-to-zero (RZ) transmitters, and generating
therefrom a corresponding forward error correction (FEC) modulated output signal in
a separate predetermined channel frequency sub-band of an overall frequency band which
includes a predetermined channel separation from an adjacent channel frequency band
generated by another RZ transmitter; (b) multiplexing the plurality of the predetermined
channel frequency bands from the plurality of RZ transmitters into separate groups
of frequency bands where the groups of frequency bands have a predetermined band-gap
separation therebetween, wherein each group of frequency bands has a predetermined
separate pre-chirp introduced before being multiplexed with all other groups of frequency
bands into a single multiplexed output signal; (c) receiving the single multiplexed
output signal in an optical transmission line which is subdivided into predetermined
optical transmission line sections; (d) receiving the single multiplexed output signal
propagating in each optical transmission line section by a separate Raman amplifier
which combines a predetermined Raman pump power signal into the optical transmission
line section in an opposite direction to the received single multiplexed output signal
to generate an output signal which is Raman amplified for increasing a path averaged
optical power without increasing nonlinear degradation; and (e) introducing dispersion
compensation from a dispersion compensating line amplifier (DCLA) into the single
multiplexed output signal from an output of a Raman amplifier at the end of a predetermined
group of optical transmission line sections for providing separate high-order dispersion
compensation for each of the groups of frequency bands in said single multiplexed
output signal.
[0013] The invention will be better understood from the following more detailed description
taken with the accompanying drawings and claims.
Brief Description of the Drawings
[0014]
FIGS. 1A and 1B show a block diagram of an exemplary ultra-long haul lightwave transmission
system in accordance with the present invention;
FIG. 2 shows a block diagram of an exemplary return-to-zero transmitter for use in
a transmitting terminal of the exemplary ultra-long haul lightwave transmission system
of FIGS. 1A and 1B in accordance with the present invention;
FIG. 3 shows a block diagram of a Raman amplifier for use in a optical transmitting
fiber of the exemplary ultra-long haul lightwave transmission system of FIGS. 1A and
1B in accordance with the present invention;
FIG. 4 graphically shows an exemplary Raman Gain characteristic shape obtainable from
the Raman amplifier of FIG. 3 in accordance with the present invention;
FIG. 5 shows a block diagram of an optical line amplifier for use in a optical transmitting
fiber of the exemplary ultra-long haul lightwave transmission system of FIGS. 1A and
1B in accordance with the present invention;
FIG. 6 shows a block diagram of a dispersion compensating line amplifier for use in
a optical transmitting fiber of the exemplary ultra-long haul lightwave transmission
system of FIGS. 1A and 1B in accordance with the present invention;
FIG. 7 shows a block diagram of an exemplary return-to-zero receiver for use in a
receiving terminal of the exemplary ultra-long haul lightwave transmission system
of FIGS. 1A and 1B in accordance with the present invention;
FIG. 8 graphically shows a plot for system performance at different pulse widths for
an exemplary system in accordance with the present invention;
FIG. 9 graphically shows a plot of channel power for an optimization of channel power
in an exemplary system in accordance with the present invention;
FIG. 10 graphically shows a contour plot for path average dispersion for one section
of a optical transmission fiber in accordance with the present invention;
FIG. 11 graphically shows a plot of channel loading penalty using a frequency band
approach in accordance with the present invention;
FIG 12 shows a graph for dispersion management of an exemplary system for an exemplary
transmission line route of 2950km consisting of mixed optical fibers in accordance
with the present invention; and
FIG. 13 graphically shows a plot for an exemplary system performance of predetermined
hybrid fiber types after transmitting optical signals through a distance of 2950km
through the mixed fibers in accordance with the present invention.
Detailed Description
[0015] The present invention uses three enabling technologies to overcome the practical
challenges found in the prior art. These three enabling technologies are: (a) a return-to-zero
(RZ) modulation format, (b) a wavelength band structure for each of bandwidth, dispersion,
and nonlinearity management, and (c) distributed Raman amplification. None of the
three enabling technologies by itself can be used to realize a required system performance
for a high capacity, long distance lightwave transmission system. The description
which follows describes the problems that must be solved and how the above-mentioned
three technologies are optimally used to achieve the required system performance.
[0016] Since optical nonlinearities and amplifier noise are the two major limitations for
ultra-long haul optical transmission, the utilization focus of the above-mentioned
three different technologies is to minimize the impact of optical nonlinearities while
maximizing the optical signal-to-noise ratio (OSNR). There are four major optical
nonlinearities in single mode transmission fibers. They are (a) self-phase modulation
(SPM) which refers to a single channel nonlinear effect resulting from an optical
Kerr effect, (b) cross-phase modulation (XPM) which is a multi-channel nonlinear effect
resulting from the optical Kerr effect, (c) four-wave mixing (FWM) which is a coherent
multi-channel nonlinear effect resulting from the optical Kerr effect, and (d) stimulated
Raman scattering (SRS) which is a multi-channel nonlinear effect resulting from an
interaction between laser radiation and molecular vibrations.
[0017] It is found that a return-to-zero (RZ) modulation format is very effective in minimizing
SPM [described in nonlinearity (a) above] if a dispersion map is optimized. There
exists a certain range of optimum pulse width which allows the design of a an RZ transmission
system using available commercial transmitter components. The optimum pulse width
is also affected by choices of frequencies for channel spacings.
[0018] With regard to cross-phase modulation (XPM) described for nonlinearity (b) above,
XPM is the most difficult optical nonlinearity to manage. In accordance with the present
invention, three techniques are used to minimize XPM. First, the transmission bandwidth
is divided into frequency bands with predetermined band gaps between adjacent bands
to aid in limiting the XPM degradation to a tolerable level. A channel loading penalty
due to XPM decays much faster than with uniform channel allocation or non-band techniques.
Second, the use of backward distributed Raman amplification helps to increase the
path averaged optical powers without increasing nonlinear degradation. This makes
it possible to maintain a required OSNR while reducing the channel launching power.
Third, as is described hereinbelow, the technique for higher dispersion management
helps to randomize a phase relationship among different frequency bands. This reduces
the worst case scenario of channel realignment due to periodic dispersion compensation.
[0019] Four-wave mixing (FWM), described for nonlinearity (c) above, is minimized by channel
frequency allocation as well as distributed Raman amplification for the reasons expressed
hereinbefore for the advantages of channel frequency allocation and distributed Raman
amplification. The impact of stimulated Raman scattering (SRS), described for nonlinearity
(d) above, is two-fold. SRS causes energy transfer among channels which generates
a dynamic power tilt, and it gives rise to statistical channel-to-channel cross-talk.
In accordance with the present invention, the dynamic power tilt problem is solved
by using automatic band power equalization inherent in the structure of higher order
dispersion management. Channel-to-channel Raman cross-talk is substantially reduced
by distributed amplification due to the use of a lower launching power.
[0020] The ultimate transmission distance is determined by optical nonlinearities and optical
amplifier noise. It is possible to control the growth of amplifier noise by placing
optical amplifiers at closer spacings, just as in submarine optical transmission systems.
However, system designers do not have this freedom in designing terrestrial transmission
systems. Even though the impacts of optical nonlinearities have been minimized, forward
error correction (FEC) is required to extend the transmission distance beyond the
nonlinearity limits. The effectiveness of FEC is affected by the extent of nonlinearity
management. On the other hand, an out-of-band FEC technique requires higher bit rate
or shorter pulses, which also affect the nonlinearity management.
[0021] Referring now to FIGS. 1A and 1B, there is shown a block diagram of an exemplary
ultra-long haul lightwave transmission system in accordance with the present invention.
FIG. 1A shows a block diagram of an exemplary transmitting terminal 10 (shown within
a dashed-line rectangle) and an exemplary ultra-long haul optical transmission line
12 in accordance with the present invention. FIG. 1B shows a block diagram of a remaining
portion of the exemplary ultra-long haul optical transmission line 12 shown in FIG.
1A, and an exemplary receiving terminal 14 (shown within a dashed-line rectangle)
for receiving the optical signals from the transmitting terminal 10 in accordance
with the present invention.
[0022] The exemplary transmitting terminal 10 comprises a plurality of X groups of N return-to-zero
transmitters (RZTX) 20a-20n each (of which only the group of RZTXs 20a1-20n1 for group
1 and the group of RZTXs 20ax-20nx are shown), a plurality of X channel multiplexers
(CHAN MUX BAND) 22a-22x (of which only channel multiplexers 22a and 22x are shown),
a plurality of X low power optical amplifiers (A) 24a-24x (of which only amplifiers
24a and 24x are shown), a plurality of X dispersion compensating elements (DCE) 26a-26x
(of which only DCEs 26a and 26x are shown), and a band multiplexer (BAND MUX) 28.
Each of the RZTXs 20a1-20n1 receives a separate input signal and converts the associated
input signal into a separate output signal in a separate frequency band of an overall
first frequency band (BAND 1). Similarly, each of the RZTXs 20ax-20nx receives a separate
input signal and converts the associated input signal into a separate output signal
in a separate frequency band of an overall last frequency band (BAND X). It is to
be understood, that although each group of RZTXs is shown as comprising n RZTXs 20a-20n,
the number "n" in each group can comprise a different number where in a practical
arrangement an equal number of RZTXs 20a-20n are not available for each of the X groups
of RZTXs 20a-20n. The channel multiplexers 22a and 22x receive the outputs from the
RZTXs 20a1-20n1 and 20ax-20nx, respectively, and multiplex the received signals within
the proper frequency band slots of the respective overall frequency bands 1 and X
and transmits a separate single output signal. The combined signals in the first overall
frequency band (BAND 1) from the first channel multiplexer (CHAN MUX BAND 1) 22a are
transmitted through a low power optical amplifier 24a, which amplifies the first overall
frequency band signal to a predetermined level, and then a dispersion compensating
element 26a that adds a predetermined chirp (dispersion value) to the associated first
overall frequency band. Similarly, the combined signals in the last overall frequency
band (BAND X) from the last channel multiplexer (CHAN MUX BAND X) 22a are transmitted
through a low power optical amplifier 24x, which amplifies the last overall frequency
band signal to a predetermined level, and then to a dispersion compensating element
26x that adds a predetermined chirp (dispersion value) to the associated last overall
frequency band. The output signals from each of the dispersion compensating elements
26a-26x are received at separate inputs of the band multiplexer 28 where the plurality
of X overall frequency bands are combined into a single output signal, from the transmitting
terminal 10 for transmission over an ultra-long haul optical transmission line 12.
[0023] In the transmitting terminal 10, the overall transmission frequency bandwidth is
divided into many smaller frequency bands 1-X with predetermined gaps therebetween
between the adjacent frequency bands. Each frequency band 1-X contains a predetermined
number "n" of channels obtained from the associated RZTXs 20a-20n with predetermined
channel spacings.
[0024] The ultra-long haul optical transmission line 12 comprises a plurality of optical
line amplifiers (OLA) 32, a plurality of backward-pumped Raman amplifiers (RA) 34,
and at least one dispersion compensating line amplifier (DCLA) 36. Each section of
the optical transmission line comprises one of the optical line amplifier (OLA) 32
at the beginning which can comprise an Erbium-doped fiber amplifier (EDFA), and one
of the backward-pumped Raman amplifiers (RA) 34 at the end of the optical transmission
line section. After a predetermined number of optical transmission line sections,
the OLA 32 is replaced by one of the dispersion compensating line amplifier (DCLA)
36. The functionalities of the DCLA 36 are mainly two-fold. The DCLA 36 compensates
for higher-order chromatic dispersion, and equalizes the band power. The DCLA 36 also
provides the benefits of reducing the cross-phase modulation (XPM) among adjacent
bands. The location of the DCLA 36 along the optical transmission line 12 can be flexible,
which allows network designers to place DCLAs 36 at convenient locations. After the
DCLA 36, the structure of the optical transmission line 12 periodically repeats itself
until reaching the final destination at the receiving terminal 14 shown in FIG. 1B.
[0025] As shown in FIG. 1B, in the last optical transmission line section, the optical signals
pass through the final backward-pumped Raman amplifier (RA) 34, and are received by
the receiving terminal 14. The receiving terminal 14 comprises a pre-amplifier (PA)
40, a broadband dispersion compensator (DC) 42, a band demultiplexer (BAND DEMUX)
44, a plurality of post dispersion compensating (POST DC) modules 46a-46x (of which
only Post DCs 46a and 46x are shown), a plurality of channel band demultiplexers (CHAN
BAND DEMUX) 48a-48x (of which only CHAN BAND DEMUXs 48a and 48x are shown), and a
plurality of (N)(X) return-to-zero receivers (RZRX) 50a-50n (of which only RZRX receivers
50a1-50an and 50n1-50nx are shown for bands 1 and X, respectively).
[0026] The band demultiplexer 44 functions to separate each-of the overall frequency bands
1-X received from the optical transmission line 12 into individual outputs for propagation
over optical fiber paths 45a-45x, respectively, of which only optical fiber paths
45a and 45x are shown with included devices. A post dispersion compensating module
46a receives the signals in frequency band 1 via optical fiber path 45a and provides
post dispersion compensation for frequency band 1. Similarly, a post dispersion compensating
module 46x receives the signals in frequency band X via optical fiber path 45x and
provides post dispersion compensation for frequency band X. Each of the plurality
of channel band demultiplexers 48a-48x receives the output signals from a separate
associated one of the plurality of post dispersion compensating modules 46a-46x, and
further demultiplexes the received frequency band (e.g., frequency band 1) into the
associated individual channels for transmission over a separate output path. Each
of the channels 1-n of frequency band 1 from channel band demultiplexer 48a is coupled
to an corresponding separate one of the plurality of RZRXs 50a1-50n1, and each of
the channels 1-n of frequency band X from channel band demultiplexer 48x is coupled
to an corresponding separate one of the plurality of RZRXs 50ax-50nx.
[0027] Referring now to FIG. 2, there is shown a block diagram of an exemplary return-to-zero
transmitter (RZTX) 20 (shown within a dashed line rectangle) for use in the transmitting
terminal 10 of the exemplary ultra-long haul lightwave transmission system of FIGS.
1A and 1B in accordance with the present invention. The RZTX 20 comprises an Forward
Error Correction (FEC) encoder 60, a modulation driver 62, a pulsed laser driver 64,
a short pulse laser source 66, and an optical modulator (MODULATOR) 68. Input electronic
data and clock signals are received at separate inputs of the FEC encoder 60 which
provides an encoded data output signal for transmission to the modulator driver 62.
The modulation driver 62 uses the FEC encoder 60 output signal to drive the optical
modulator 68. The electronic clock signal is also received at an input of the pulsed
laser driver 64 which is used to drive the short pulse laser source 66 to provide
a corresponding optical laser output signal. The short pulse laser source 66 can be
implemented by any suitable laser source as, for example, a Lithium-Niobate modulator,
and electro-absorption modulator, or directly by a mode-locked short pulse laser.
The optical modulator 68 modulates the optical output signal from the short pulse
laser source 66 with the FEC encoded output signal from the modulation driver 62 to
generate an return-to-zero (RZ) optical data output signal from the RZTX 20.
[0028] Referring now to FIG. 3, there is shown a block diagram of a Raman amplifier (RA)
34 (shown within a dashed line rectangle) for use in a optical transmitting fiber
12 of the exemplary ultra-long haul lightwave transmission system of FIGS. 1A and
1B in accordance with the present invention. The Raman amplifier 34 comprises a wavelength
division multiplexer (WDM) 70, a plurality of n Raman pump lasers (RAMAN PUMP) 72a-72n
(of which only Raman pump lasers 721, 72b, and 72n in FIG. 3 are shown), and a pump
laser combiner (COMB.) 74. The number of Raman pump lasers 72a-72n, as well as the
pump wavelengths therefrom, is determined by the types of fibers of the optical transmission
fiber 12 and the required amplifications. The combined output from the pump laser
combiner 74 is coupled into the optical transmission line 12 in the opposite direction
to the received input signals by the WDM 70.
[0029] Referring now to FIG. 4, there is shown a graph of Gain in dB (decibels) on the Y-axis
versus Wavelength in nanometers (nm) on the X-axis of an exemplary Raman Gain characteristic
shape for various channels obtainable from the Raman amplifier of FIG. 3 in accordance
with the present invention. The exemplary Raman Gain characteristic shape of FIG.
4 is obtained using a transmission fiber 12 which is a non-zero dispersion-shifted
fiber (NZDSF)-with a length of 100km, a signal loss of 0.22dB/km, a pump loss of 0.3dB/km,
and two pump lasers 72 with a total pump power of 247.4 mW. The backward distributed
Raman amplifications have the benefits of optical signal-to-noise ratio (OSNR) enhancement
and negligible nonlinear degradations. Due to the attenuation of the optical fiber
12, the signal power near the end of the transmission fiber 12 is orders of magnitude
lower than that at the input. Some moderate amplification near the end of the transmission
fiber 12 will not increase the signal power to the nonlinear regime. On the other
hand, the path averaged signal power is greatly enhanced so that the overall noise
build-up of the transmission line 12 is suppressed. The selection of the Raman gain
is determined by both nonlinear degradation and extra noise addition due to the Raman
amplification. After Raman amplification, the output signals from the Raman amplifier
34 of FIG. 3 are coupled into the line amplifiers 34 of a next section of the transmission
line 12.
[0030] Referring now to FIG. 5, there is shown a block diagram of an exemplary optical line
amplifier (OLA) 32 (shown within a dashed line rectangle) for use in a optical transmitting
fiber 12 of the exemplary ultra-long haul lightwave transmission system of FIGS. 1A
and 1B in accordance with the present invention. The exemplary optical line amplifier
32 comprises a serial arrangement of a low noise pre-amplifier (PA) 80, such as an
Erbium-doped fiber amplifier (EDFA), a gain equalization filter (GAIN EQUAL. FLT.)
82, an optional broadband dispersion compensator 84 (shown within a dashed line rectangle),
and a boost amplifier (BA) 86. The gain equalization filter 82 functions to equalize
gain variations resulting from a Raman amplifier 34 at the end of a prior section
of the transmission line 12, the transmission fiber 12 itself, the pre-amplifier 90,
and the boost amplifier 85 (shown in FIG. 5) in the prior optical line amplifier 32.
The broadband dispersion compensator 84 is optional depending on the types of the
transmission fibers used in the transmission lines 12. The broadband dispersion compensator
84 is required for standard non-dispersion shifted fibers that have high chromatic
dispersion at the transmission wavelength, while it is not required for the non-zero
dispersion-shifted fibers (NZDSF).
[0031] Referring now to FIG. 6, there is shown a block diagram of an exemplary dispersion
compensating line amplifier (DCLA) 36 (shown within a dashed line rectangle) for use
in a optical transmitting fiber 12 of the exemplary ultra-long haul lightwave transmission
system of FIGS. 1A and 1B in accordance with the present invention. The dispersion
compensating line amplifier (DCLA) 36 comprises a pre-amplifier (PA) 90, a gain equalization
filter (GEF) 92, a broadband dispersion compensator (BDC) 94, a band demultiplexer
96, a plurality of X dispersion compensating modules (DCM) 98a-98x (of which only
DCMs 98a, 98b, and 98x are shown), a plurality of X band power equalizers (BPE) (of
which only BPEs 99a, 99b, and 99x are shown), a band multiplexer (BAND MUX) 100, and
a boost amplifier (BA) 102. The dispersion compensating line amplifier 36 replaces
an optical line amplifier 32 after a predetermined sections of the optical transmission
line 12. The arrangement of the pre-amplifier (PA) 90, gain equalization filter (GEF)
92, and broadband dispersion compensator (BDC) 94 is similar to that found for the
PA 80, GEF 82, and BDC 84 of the optical line amplifier (OLA) 32 of FIG. 5, but differs
in that a higher order of dispersion compensation is provided by the BDC 94. The output
power of the pre-amplifier 90 is designed to be small so that nonlinearities of the
BDC 94 are insignificant. Another difference between the DCLA 36 and the OLA 32 is
that the BDC 94 is required regardless of the fiber types found in the optical transmission
line 12.
[0032] The output of the BDC 94 is received in the band demultiplexer 96 which divides the
overall received frequency band into the X frequency bands therein and outputs the
1-X frequency bands over separate optical paths 97a-97x (of which only paths 97a,
97b, and 97x of FIG. 6 are shown). Optical path 97a receives the frequency band 1
and includes a serial arrangement of the dispersion compensating module (DCM) 98a
and the band power equalizer (BPE) 99a. The DCM 98a includes a predetermined dispersion
value which is specific to the frequency band 1. The combination of the BDC 94 and
the DCM 99a makes it possible to design the accumulated dispersion value for frequency
band 1 to a predetermined value. Each of the DCMs 98b-98x and the associated BPEs
99b-99x function in a similar manner for the associated frequency bands 2-X, respectively.
Due to the higher-order dispersion, or dispersion slope of the transmission fiber
12 and the BDC 94, the accumulated dispersion of each of the frequency bands 1-X will
differ. Using a separate DCM 98a-98x for each of frequency bands 1-X negates this
difference and provides a flexible mechanism for dispersion management for any type
of optical fiber that is used for optical transmission fiber 12. The output of each
of the DCMs 98a-98x is coupled to a separate associated on of the BPEs 99a-99x which
can be, for example, an adaptive variable attenuator. The output from each of the
BPEs 99a-99x is coupled to a separate input of the band multiplexer 100 where the
1-X frequency band signals are combined into a single output signal which is amplified
by the boost amplifier 102 to a predetermined value. The implementation of the DCLA
36 can vary depending on other considerations such as cost, size, and loss. For example,
the band multiplexer 100 and the band demultiplexer 96 can by formed from dielectric
thin film filters, or an interleaver and filter combination.
[0033] Referring now to FIG. 7, there is shown a block diagram of an exemplary return-to-zero
receiver (RZRX) 50 (shown within a dashed line rectangle) for use in a receiving terminal
14 of the exemplary ultra-long haul lightwave transmission system of FIGS. 1A and
1B in accordance with the present invention. The RZRX 50 comprises an optical automatic
gain control (OAGC) unit 110, a standard linear channel comprising a high-speed PIN
diode (PIN) 112, a low noise amplifier (LNA) 114, and a low-pass filter (FILTER) 116.
The RZRX 50 further comprises a clock/data recovers (CDR) unit 118, a forward error
correction (FEC) decoder 120, a peak power detector (PEAK DETECTOR) 122, a OAGC feedback
mechanism (OAGC FEEDBACK) 124, and an OAGC driver 126. An associated incoming optical
channel signal is received in the OAGC unit 110 and amplified in an EDFA therein,
and then coupled into the high-speed PIN diode 112 to complete a conversion of the
received optical signal into a corresponding electrical signal output signal. The
LNA 114 amplifies the electrical output signal from the PIN diode 112 and passes it
through the low-pass filter 116. The filtered output signal is transmitted to the
CDR 118 and the peak power detector 122.
[0034] The CDR 118 recovers the clock and data signal from the received channel signal and
provides them as an input to the FEC decoder 120. The FEC decoder corrects for any
transmission errors in the data signal using the FEC information, and provides the
corrected data signal as an output from the RZRX 50. The output of the peak power
detector 122 has a predetermined bandwidth and is used as a feedback signal to the
OAGC feedback unit 124 and, in turn, the OAGC driver 126. The OAGC driver 126 provides
a feedback signal to the OAGC unit 110 which is used to control the EDFA pump current
so that the peak power at the CDR 118 is a fixed value.
[0035] There are four important system parameters that have the biggest impact on system
performance. These are (a) pulse width, (b) values of pre-chirp, (c) path averaged
dispersion, and (d) channel power. Since the bit error rate (BER) or Q factor are
the ultimate indicator for system performance, Q or BER is used to optimize the system
performance. Since Q is a highly nonlinear function of not only the four parameter
mentioned hereinabove, but also many other system parameters, a large number of system
simulations are required so that a global optimization is achieved. An example of
multidimensional Q mapping are summarized as follows.
[0036] In an design for a exemplary system to describe the present invention, it is assumed
that there are a total of 56 channels grouped into 14 bands with each band comprising
four channels with a channel separation of 50 GHz. Although an information bit rate
is 10 Gbit/s, the actual bit rate is increased to 12.12 Gbit/s due to extra bandwidth
required from the FEC encoder 60 shown in FIG. 2. The bandgap is 150 GHz. The transmission
fiber 12 is the standard non-dispersion shifted fiber (NDSF) with a span length of
100km. The Raman gain after gain equalization is 8dB.
[0037] Referring now to FIG. 8, there is graphically shown a plot for an optimization of
pulse width for improved system performance with different pulse widths in picoseconds
(ps) shown along the horizontal axis versus Q in decibels (dB) along the vertical
axis for the exemplary system. When the pulse width is shorter than 20ps, the bandwidth
of each channel is so large that significant spectral overlapping occurs between adjacent
channels, which gives rise to system degradation. For pulse widths larger than 35ps,
a "walk-off" distance and the dispersion length increases, giving rise to a higher
nonlinear penalty. For the hereinabove described exemplary system, a pulse width of
around 25ps is found to be an optimal value.
[0038] Referring now to FIG. 9, there is shown a plot of channel power in units of dBm (decibels/milliwatt)
in FIG. 9 along the X-axis versus Q in dB along the Y-axis for the exemplary system.
For determining optimization of channel power, the amplifier noise dominates for a
channel power below 2dBm, and nonlinearity dominates for a channel power greater than
5dBm. Therefore, the optimal channel power for the above exemplary system is around
2-4 dBm.
[0039] Referring now to FIG. 10, there is graphically shown a contour plot for path average
dispersion in units of ps/nm/km along the X-axis versus pre-chip in a unit normalized
value on the Y-axis for one section of the optical transmission fiber 12. Q contours
are plotted in 1dB intervals. The numbers 11-19 provided for the various contour lines
in FIG. 10 in 1dB increments and represent corresponding Q factor values which are
monitor system performance. There is a 1:1 correspondence between the Q factor and
the bit error rate (BER). Therefore, if the Q factor is 18 the BER is 10
- 15. There is a quasi-linear relationship between the pre-chirp and the average dispersion.
The optimum combination of pre-chirp and average dispersion is when the pre-chirp
= 0.4, and the average dispersion = 0.3 ps/nm/km. There is a large available margin
around the optimum point so that the 1dB range for pre-chirp is from 0.26-0.55, while
for the average dispersion it is from 0.15-0.5. It is shown that the exemplary system
has a significant tolerance to both pre-chirp and average dispersion. Since the optimization
is done using multiple channels, the results are different from those of dispersion
managed soliton (DMS) systems. The channel power is selected in such a way that the
system behavior is quasi-linear. The impacts of self-phase modulation (SPM), cross-phase
modulation (XPM), four-wave mixing (FWM), and Raman effects are minimized in the present
invention in such a manner that the total system characteristics are similar to those
of linear systems. There are fundamental difference between the present inventive
system and prior art return-to-zero (RZ) systems such as dispersion-managed soliton
(DMS) systems. For example, DMS predicts significant power enhancement, which is valid
for single channel propagation. It further requires accurate balance between the SPM
in a transmission fiber and SPM in a dispersion compensating fiber, which often results
in a much smaller system margin. In accordance with the present invention, the power
in the dispersion compensating elements of (a) the broadband dispersion compensator
(BDC) 84 and 94 in the optical line amplifiers 32 and the transmitter terminal 14,
and (b) the dispersion compensating modules (DCM) 98a - 98x in the dispersion compensating
line amplifier 36 are designed to be smaller than the nonlinear threshold. Therefore,
the spectral broadening due to SPM is balanced by a proper design of the pre-chirp.
This approach has an advantage over DMS systems in that it enlarges the system margin
as is shown in FIG. 10. Another advantage is that it allows a system designer to deal
with any types of transmission fibers using the same principles.
[0040] Referring now to FIG. 11, there is graphically shown a plot of channel loading penalty
using a frequency band approach where the channel number (Channel #) is shown along
the X-axis and Q in units of dB (decibels) are shown along the Y-axis for a cross-phase
modulation (XPM) effect. The band structure has an advantage of minimizing the nonlinear
channel-to-channel interaction which is always a big concern for network designers.
Scalability provides significant economic and network flexibility advantages. From
the plot of FIG. 11, the nonlinear degradation is limited to a few channels (e.g.,
channels 4-9) since they are outside an allowable channel loading penalty of 0.5 dB.
The total channel loading penalty is controlled to within 0.5 db, and the absolute
values of Q are lower in value than found, for example, in FIGS. 8 and 9 since the
loss in a transmission line section is much higher (25dB) and the distance is 3200km.
[0041] Referring now to FIG 12, there is shown a graph of propagation distance in kilometers
(km) along the X-axis versus Time in picoseconds (ps) along the Y-axis for an exemplary
transmission line route of 2950km consisting of mixed optical fibers designated LS
and TW. Both of the LS and TW type optical fibers are commercial non-zero dispersion-shifted
fibers (NZDSF). For the exemplary system described hereinabove, there are three sections
of the LS type fiber and two sections of the TW type fiber shown above the graph.
The length of each box at the top of FIG. 12 indicated the locations of the sites
of the DCLAs 36 along the optical transmission line 12. The pre-chirp and average
dispersion is optimized according to the design rules of the present invention. The
contour of the plot indicates the evolution of pulse intensity along the transmission
line 12. Although the optical fiber characteristics change dramatically at the interfaces
of the different fibers (LS and TW), the pulse dynamics shown by the contours in the
graph manages to evolve smoothly.
[0042] Referring now to FIG. 13, there is graphically shown an exemplary system performance
of the hybrid fiber types after transmitting signals through a distance of 2950km
in the mixed fibers of FIG. 12. The X-axis indicates the Frequency band number, and
the Y-axis indicates Q in dB. Since a forward error correction (FEC) threshold is
approximately 10dB, there is a significant system margin at a distance of nearly 3000km.
The pre-chirp is approximately 0.5, the average dispersion is approximately 0.25ps/nm/km,
and the channel power is 0 dBm. The plot 130 represents values for TW plus LS type
optical fibers while the plot 132 represents values for an FEC limit.
[0043] In summary, as a result of proper management of both optical nonlinearities such
as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM),
stimulated Raman scattering, and higher-order chromatic dispersion, a lightwave system
can be designed with the following advantages. The system can provide ultra-long haul
transmission over arbitrary single mode transmission fibers, or mixed fiber types,
without the use of electronic regenerators. The system can provide flexible channel/band
add/drop capability in the sense that an arbitrary number of channels can be dropped
or added at arbitrary locations along an optical transmission line 12. The present
invention provides a scalable network design that is enabled by a dispersion management
technique. Since the chromatic dispersion is managed on a link-by-link basis, the
transmission is not distance-dependent, which makes the network scalable. Finally,
a scalable transmission capacity is enabled by a wavelength management technique.
The three enabling technologies used to obtain the present invention are (a) a return-to-zero
(RZ) modulation format, (b) a wavelength band structure for bandwidth management,
dispersion management, and nonlinearity management, and (c) distributed Raman amplification.
The wavelength bandwidth structure includes the transmission multiplexing technique,
the structure of the dispersion compensating line amplifier (DCLA) 36 for higher-order
dispersion management, and the demultiplexing in the receiver terminal 14.
[0044] It is to be appreciated and understood that the specific embodiments of the present
invention described hereinbefore are merely illustrative of the general principles
of the invention. Various modifications may be made by those skilled in the art which
are consistent with the principles set forth.
1. An optical transmission system comprising:
a transmitter terminal comprising:
a plurality of return-to-zero (RZ) transmitters, each transmitter being adapted to
receive a separate channel input signal and to generate therefrom a corresponding
forward error correction (FEC) modulated output signal in a separate predetermined
channel frequency sub-band of an overall frequency band which includes a predetermined
channel separation from an adjacent channel frequency band generated by another RZ
transmitter; and
a multiplexing arrangement for multiplexing the plurality of the predetermined channel
frequency sub-bands from the plurality of RZ transmitters into separate groups of
frequency bands where the groups of frequency bands have a predetermined band-gap
separation therebetween wherein each group of frequency bands has a predetermined
separate pre-chirp introduced before being multiplexed with all other groups of frequency
bands into a single multiplexed output signal; and
an optical transmission line having an input coupled to an output of the multiplexing
arrangement and being subdivided into a plurality of optical transmission line sections,
said optical transmission line comprising:
a plurality of Raman amplifiers, one of the plurality of Raman amplifiers being located
at the end of each optical transmission line section and being adapted to receive
at an input thereof the single multiplexed output signal propagating in an associated
transmission line section, and to combine a predetermined Raman pump power signal
into the optical transmission line section in an opposite direction to the received
single multiplexed output signal to generate at an output thereof an output signal
which is Raman amplified for increasing a path averaged optical power without increasing
nonlinear degradation; and
at least one dispersion compensating line amplifier (DCLA) coupled to an output of
a Raman amplifier at the end of a predetermined group of optical transmission line
sections, said DCLA being adapted to introduce dispersion compensation for the single
multiplexed output signal, and to introduce separate high-order dispersion compensation
for each of the groups of frequency bands therein.
2. The optical transmission system of claim 1 wherein each RZ transmitter comprises:
an FEC encoder for receiving a separate channel input signal and generating therefrom
a corresponding forward error correction (FEC) encoded output signal;
a laser comprising a predetermined frequency which is driven by a predetermined clock
signal for generating an optical output signal;
a modulator for modulating the optical output signal from the laser with the FEC encoded
output signal from the FEC encoder for generating a channel output signal in the separate
predetermined channel frequency sub-band.
3. The optical transmission system of claim 1 wherein the multiplexing arrangement comprises:
a plurality of channel multiplexers, each channel multiplexer multiplexing a predetermined
group of channel frequency sub-bands from a separate group of RZ transmitters into
a separate single output frequency band with a predetermined bandgap from an adjacent
single output frequency band from another channel multiplexer,
a plurality of dispersion compensating arrangements, each dispersion compensating
arrangement introducing a separate predetermined pre-chirp into the single output
frequency band from a separate one of the plurality of channel multiplexers and generating
a dispersion compensated frequency band output signal; and
a band multiplexer for combining each of the dispersion compensated frequency band
output signal into the single multiplexed output signal from the transmitter terminal.
4. The optical transmission system of claim 1 wherein the Raman amplifier comprises:
a plurality of Z Raman pump lasers where Z is dependent on a type of optical fiber
used for the associated optical transmission line section, each Raman pump laser generating
an output signal at a separate predetermined wavelength;
an optical combiner for combining the output signals from the plurality of Raman pump
lasers into a single output signal; and
a wavelength division multiplexer (WDM) for receiving the single multiplexed output
signal propagating in a first direction along the associated transmission line section
for coupling the output signal from the optical combiner into the optical transmission
line in an opposite direction from said received single multiplexed output signal.
5. The optical transmission system of claim 1 wherein the dispersion compensating line
amplifier (DCLA) comprises:
a band demultiplexer for demultiplexing the single multiplexed output signal from
the transmitter terminal received from prior sections of the optical transmission
line into the separate groups of frequency bands for transmission over separate output
paths;
a plurality of dispersion compensating modules (DCM), each DCM is located in a separate
output path from the band demultiplexer for providing dispersion compensation for
the associated group of frequency bands demultiplexed onto said output path and providing
a band dispersion compensated output signal; and
a band multiplexer for combining the band dispersion compensated output signals from
the plurality of DCMs into a single multiplexed output signal from the DCLA.
6. The optical transmission system of claim 5 wherein the dispersion compensating line
amplifier (DCLA) further comprises:
a gain equalizing filter (GEF) for equalizing gain variations in the single multiplexed
output signal from the transmitter terminal occurring in prior sections of the optical
transmission line and generating an gain equalized output signal; and
a broadband dispersion compensator (BDC) for compensating for chromatic dispersion
at wavelengths in the gain equalized output signal optical transmission line received
gain equalized output signal from the gain equalizing filter and providing a dispersion
compensated output signal to the band demultiplexer.
7. The optical transmission system of claim 1 wherein the optical transmission line further
comprises a plurality of optical line amplifiers (OLA), each OLA located at the start
of a separate one of each optical transmission line section except where a DCLA is
located and comprising:
a gain equalizing filter for equalizing gain variations in the single multiplexed
output signal from the transmitter terminal occurring in a prior section of the optical
transmission line and generating an gain equalized output signal; and
an amplifier for generating an output signal from the OLA which amplifies the gain
equalized output signal to a predetermined amplification level.
8. The optical transmission system of claim 1 wherein the OLA further comprises a broadband
dispersion compensator when a non-dispersion shifted optical fiber with a high chromatic
dispersion at a predetermined transmission wavelength is used in a prior section of
the optical transmission line.
9. The optical transmission system of claim 1 further comprising a receiver terminal
located at an end of the optical transmission line opposite the location of the transmission
terminal, the receiver terminal comprising:
a band demultiplexer responsive to a received single multiplexed output signal from
the transmitter terminal for demultipexing the groups of frequency bands so that each
group of frequency bands is provided as a separate output signal onto a separate output
path thereof;
a plurality of post dispersion compensators (PDC), each PDC being located in a separate
output path from the band demultiplexer for providing separate dispersion compensation
to the an associated group of frequency bands received from the band demultiplexer;
a plurality of channel band demultiplexers, each channel band demultipexer receiving
a separate group of frequency bands from the band demultiplexer and demultiplexing
the channels in said separate group of frequency bands into individual channel output
signals for propagation along a separate output path thereof; and
a plurality of return-to-zero receivers (RZRX), each RZRX receiving a separate channel
output signal from the plurality of channel band demultiplexers and decoding data
in said channel output signal for generating an output signal from the transmitter
terminal.
10. The optical transmission system of claim 9 wherein each RZRX comprises:
an automatic gain control (AGC) arrangement responsive an associated separate channel
output signal received from the plurality of channel band demultiplexers for generating
an output signal corresponding to the received associated separate channel output
signal which is maintained at a predetermined level;
a clock/data recovery unit for recovering a clock signal and a data signal from the
output signal from the AGC arrangement; and
a forward error correction (FEC) decoder responsive to the clock signal and data signal
recovered by the clock/data recovery unit for decoding the data and generating a decoded
data output signal from the RZRX.
11. A method of transmitting signals in an optical transmission system comprising the
steps of:
(a) receiving each channel input signal of a plurality of channel input signals in
a separate one of a plurality of return-to-zero (RZ) transmitters, and generating
therefrom a corresponding forward error correction (FEC) modulated output signal in
a separate predetermined channel frequency sub-band of an overall frequency band which
includes a predetermined channel separation from an adjacent channel frequency band
generated by another RZ transmitter;
(b) multiplexing the plurality of the predetermined channel frequency bands from the
plurality of RZ transmitters into separate groups of frequency bands where the groups
of frequency bands have a predetermined band-gap separation therebetween, wherein
each group of frequency bands has a predetermined separate pre-chirp introduced before
being multiplexed with all other groups of frequency bands into a single multiplexed
output signal;
(c) receiving the single multiplexed output signal in an optical transmission line
which is subdivided into predetermined optical transmission line sections;
(d) receiving the single multiplexed output signal propagating in each optical transmission
line section by a separate Raman amplifier which combines a predetermined Raman pump
power signal into the optical transmission line section in an opposite direction to
the received single multiplexed output signal to generate an output signal which is
Raman amplified for increasing a path averaged optical power without increasing nonlinear
degradation; and
(e) introducing dispersion compensation from a dispersion compensating line amplifier
(DCLA) into the single multiplexed output signal from an output of a Raman amplifier
at the end of a predetermined group of optical transmission line sections for providing
separate high-order dispersion compensation for each of the groups of frequency bands
in said single multiplexed output signal.
12. The method of claim 11 wherein in performing step (a) performing the substeps of:
(a1) receiving a separate channel input signal in an FEC encoder for generating therefrom
a corresponding forward error correction (FEC) encoded output signal;
(a2) generating an optical output signal from a laser comprising a predetermined frequency
which is driven by a predetermined clock signal;
(a3) modulating the optical output signal from the laser in step (a2) with the FEC
encoded output signal from the FEC encoder in step (al) for generating a channel output
signal in the separate predetermined channel frequency sub-band.
13. The method of claim 11 wherein in performing step (b) performing the substeps of:
(b1) multiplexing a separate predetermined group of channel frequency bands from a
separate group of RZ transmitters in a separate one of a plurality of channel multiplexers
for generating a separate single output frequency band with a predetermined bandgap
from an adjacent single output frequency band from another channel multiplexer;,
(b2) introducing a separate predetermined pre-chirp into the single output frequency
band from a separate one of the plurality of channel multiplexers in step (bl) in
a separate one of a plurality of dispersion compensating arrangements and generating
a dispersion compensated frequency band output signal; and
(b3) combining each of the dispersion compensated frequency band output signal into
the single multiplexed output signal from the transmitter terminal in a band multiplexer.
14. The method of claim 11 wherein in performing step (d) performing the substeps of:
(d1) generating an output signal at a separate predetermined wavelength in each of
a plurality of Z Raman pump laser, where Z is dependent on a type of optical fiber
used for the associated optical transmission line section;
(d2) combining the output signals from the plurality of Raman pump lasers into a single
output signal in an optical combiner; and
(d3) receiving the single multiplexed output signal propagating in a first direction
along the associated transmission line section in a wavelength division multiplexer
(WDM) for coupling the output signal from the optical combiner from step (d2) into
the optical transmission line in an opposite direction from said received single multiplexed
output signal.
15. The method of claim 11 wherein in performing step (e) performing the substeps of:
(e1) demultiplexing the single multiplexed output signal from the transmitter terminal
received from prior sections of the optical transmission line into the separate groups
of frequency bands for transmission over separate output paths in a band demultiplexer;
(e2) providing dispersion compensation for an associated group of frequency bands
demultiplexed onto an associated output path in step (e1) in a separate one of a plurality
of dispersion compensating modules (DCM), and providing a band dispersion compensated
output signal from said separate DCM; and
(e3) combining the band dispersion compensated output signals from the plurality of
DCMs into a single multiplexed output signal from the DCLA in a band multiplexer .
16. The method of claim 5 wherein in performing step (e2) performing the steps of:
(f) equalizing gain variations in the single multiplexed output signal from the transmitter
terminal occurring in prior sections of the optical transmission line in a gain equalizing
filter (GEF), and generating an gain equalized output signal; and
(g) for compensating for chromatic dispersion at wavelengths in the gain equalized
output signal optical transmission line received gain equalized output signal from
the gain equalizing filter in a broadband dispersion compensator (BDC), and providing
a dispersion compensated output signal to the band demultiplexer of step (e1).
17. The method of claim 11 wherein in performing step (c) the optical transmission line
comprises a plurality of optical line amplifiers (OLA), each OLA located at the start
of a separate one of each optical transmission line section except where a DCLA of
step (e) is located, performing the substeps of:
(c1) equalizing gain variations in the single multiplexed output signal from the transmitter
terminal occurring in a prior section of the optical transmission line in a gain equalizing
filter and generating an gain equalized output signal; and
(c2) generating an output signal from the gain equalized output signal of step (c1)
in an amplifier for amplifying the gain equalized output signal to a predetermined
amplification level.
18. The method of claim 11 wherein a receiver terminal is located at an end of the optical
transmission line opposite the location of the transmission terminal, the method comprising
the further step of:
(f) demultipexing the groups of frequency bands received in the single multiplexed
output signal from the transmitter terminal in a band demultiplexer of a receiver
terminal for directing each group of frequency bands as a separate output signal onto
a separate output path thereof;
(g) introducing separate dispersion compensation to the an associated group of frequency
bands received from the band demultiplexer in a separate one of a plurality of post
dispersion compensators (PDC) located in a separate output path from the band demultiplexer;
(h) demultiplexing channels in a separate group of frequency bands from the band demultiplexer
into individual channel output signals in a separate one of a plurality of channel
band demultiplexers for propagation along a separate output path thereof; and
(i) receiving each separate channel output signal from the plurality of channel band
demultiplexers in a separate one of a plurality of return-to-zero receivers (RZRX),
and decoding data in said channel output signal for generating an output signal from
the transmitter terminal.
19. The method of claim 18 wherein in performing step (i) performing the substeps of:
(j) generating an output signal in an automatic gain control (AGC) arrangement of
the RZRX corresponding to an associated separate channel output signal received from
the plurality of channel band demultiplexers which is maintained at a predetermined
level;
(k) recovering a clock signal and a data signal from the output signal from the AGC
arrangement in a clock/data recovery unit; and
(l) decoding the data using the clock signal and data signal recovered by the clock/data
recovery unit in step (k) in a forward error correction (FEC) decoder for generating
a decoded data output signal from the RZRX.